The Pittsburgh Southern Railway was a railroad in the U.S. state of Pennsylvania. It was formed in March 1879 by the merger of the 3 ft (914 mm) narrow gauge Pittsburgh Southern Railroad (which was the 3 ft 4 in (1,016 mm) narrow gauge Pittsburgh, Castle Shannon and Washington Railroad from July 1877 to April 1878), , and . It ran from Washington to Castle Shannon, where it connected to the Pittsburgh and Castle Shannon Railroad. An attempt to use the 4 ft 8+1⁄2 in (1,435 mm) standard gauge Little Saw Mill Run Railroad as a substitute connection to Pittsburgh using dual gauge track led to the Castle Shannon Railroad War of 1878.